An apparatus which records information on a recording medium at high density and reproduces it by the use of a laser beam condensed to the submicron order, can record large capacities of information such as images and sounds and is expected to become an apparatus indispensable to the future information-oriented society. Such an optical recording and reproducing apparatus is, for example, an optical disc device. The optical disc devices include three types; (1) the reproduce-only type, (2) the add-on type and (3) the rewritable type, and the reproduce-only type and the add-on type have substantially reached practical stages.
On the other hand, the so-called rewritable type capable of readily erasing information recorded once and recording new information does not have an established method yet. Therefore, developments are being actively promoted for optical recording and reproducing apparatuses capable of recording, reproduction and erasure with the same extent of ease as that of tape recorders.
A recording medium which can accomplish this theme is, for example, one employing an amorphous semiconductor as a recording material as described in U.S. Pat. No. 3,530,441 entitled "Method and Apparatus for storing and retrieving information" issued on Sept. 22, 1970. However, an amorphous state cannot be said an essentially stable state and exhibits a trend toward crystallization when let stand, and it has not been suitable for a memory over a long term.